If I remember correctly, there is a very good example in the SRG installation and configuration guide.
Basically, you are configuring the PBX to replace AC1 and/or AC2 with some other digit(s) when dialed from IP phones in a particular zone.
The digits that replace AC1 and/or AC2 are then configured as steering codes pointing to your virtual route. Entries in the NRS point those steering codes to an endpoint, where the leading digits are stripped off, and the call is sent the the PSTN using local facilities. The system where the call leaves your network is usually the system that is local to the IP phones.